There are cruise control systems with which the speed of the vehicle may be regulated at a desired speed selected by the driver for vehicles. In addition, the distance from a vehicle in front is measurable with the help of a distance sensor, e.g., a radar or lidar sensor. The cruise control is then adjusted to maintain a predetermined distance, which may be as a function of speed, from the vehicle traveling in front, which has been selected as the target object. Such systems are also referred to as adaptive cruise control or ACC systems.
Satisfactory functioning of an ACC system presupposes that the distance sensor is functioning faultlessly and is correctly adjusted so that vehicles traveling in front in one's own lane may be located reliably. Apart from a disturbance in function or a misadjustment of the distance sensor, environmental factors such as heavy rain or snowfall, soiling of the sensor head or the like may result in a reduction in sensor range even to the extent of the distance sensor becoming blind. In general, the function of radar sensors is less impaired than that of lidar sensors by rain or snowfall, but even with radar sensors there is the risk of the sensor being blinded in particular when the radar antenna mounted in an exposed position on the front of the vehicle becomes encrusted with snow, ice or sludge thrown up from the road. For safety reasons, it is desirable for the blindness of a distance sensor to be detected automatically and then for a shutdown of the ACC system to be triggered and an acoustic or visual warning to be output to the driver.
German patent document no. 196 44 164 A1 refers to and discusses an automotive radar system in which a dielectric body which is used for focusing the radar waves is covered with printed electric conductors which allow detection of a layer of dirt, snow or moisture that interfere with radar reception. If necessary, the interfering deposit may also be removed automatically, e.g., by electric heating or by a type of wiper. However, such monitoring and cleaning systems are relatively complex and expensive.
German patent document no. 199 45 268 discusses and refers to a method which provides for recognizing blindness of a radar sensor merely on the basis of the signals supplied by the radar sensor itself. By analyzing these signals, various indicators are formed which indicate blindness of the sensor with a relatively high reliability, depending on the driving situation. Examples of suitable indicators include the angle quality of all objects detected by the radar system, this quality describing the object's stability, i.e., the rate of detection failures of the target object selected for the regulation, the average power of the signals received by the sensor, the sum of all objects detected by the system during a measurement, the relationship between the object distance and the radar amplitude for the object detected at the greatest distance from one's own vehicle and the presence or absence of road reflection. These indicators are weighted with weighting factors which may in turn be variable, depending on the traffic situation, and a decision signal indicating whether or not the sensor is blind is obtained by comparing the resulting weighted sum of the indicators with a threshold value.
With regard to the traffic situation, a distinction between following and driving freely is relevant for detection of blindness. Following refers to a traffic situation in which a vehicle traveling directly in front is selected as a target object and is followed at an appropriated distance. In this situation, a sudden loss of the target object is an important indicator of blindness. Driving freely refers to a traffic situation in which no relevant target object is selected but instead the cruise control relies on the desired speed selected by the driver. However, even in this situation, the sensor will in general also detect other radar targets, e.g., street reflection, movable targets or stationary targets at the edge of the road, vehicles in other lanes and the like. The radar echoes of these targets may then be used for detection of blindness.
When blindness of a sensor is detected with such a system, whether driving freely or following, this results directly in shutdown of the ACC system and the driver is prompted by a takeover instruction to take control of the longitudinal movement of the vehicle himself using the accelerator pedal and brake.